1. Field of the Invention
This invention relates to an electrophotographic apparatus such as a copying machine or printer of the electrophotographic type, and particularly to an electrophotographic apparatus having pre-exposure means for eliminating the residual charges of a photosensitive member.
2. Related Background Art
FIG. 6 of the accompanying drawings schematically shows the construction of an image forming apparatus which is an electrophotographic apparatus according to the prior art. This image forming apparatus is provided with a photosensitive drum 1 which is a photosensitive member rotated in the direction of arrow A, and a photoconductive layer on the surface of this photosensitive drum 1 is uniformly charged by the primary current of a primary charger 2, whereafter it is subjected to the exposure 3 of the optical image of an original and an electrostatic latent image is formed on the surface of the photosensitive drum 1.
The optical image is such that an original placed on original supporting glass 101 is irradiated by a halogen lamp 103 and a reflecting plate 104, and the reflected optical image comes into a zoom lens 110 via reflecting mirrors 105, 102 and 107. It is further irradiated (applied) to the photosensitive drum 1 via reflecting mirrors 106, 108 and 109. At this time, the image is reduced or enlarged by the positional relation between the reflecting mirror 107 and the zoom lens 110.
The electrostatic latent image thus formed comes to a position opposed to a developing device 4 with the rotation of the photosensitive drum 1, and is developed by a toner supplied from the developing device 4 and is visualized as a toner image.
The thus obtained toner image advances to a transfer portion in which a transfer charger 6 is disposed as the photosensitive drum 1 is rotated. On the other hand, a transfer material P moved in synchronism with the photosensitive drum 1 is guided to a conveying path 5 and comes to the transfer portion, where the transfer material P is brought into close contact with the surface of the photosensitive drum 1.
At this position, charges of the opposite polarity to the toner are imparted to the back of the transfer material P by the transfer charger 6, and the toner image on the photosensitive drum 1 is shifted and adsorbed onto the transfer material P by an electrostatic force and is thus transferred.
A separating charger 7 is installed on the downstream side of this transfer charger 6 with respect to the direction of movement of the transfer material. The transfer material P to which the toner image has been transferred has its charges lost by the corona discharge by the separating charger 7 and loses its adsorption force to the photosensitive drum 1, and is separated from the photosensitive drum 1 by the help of the elasticity of the transfer material P itself and further the deadweight (gravity) of the transfer material P.
Also, any toner not transferred to the transfer material P and left on the photosensitive drum 1 is scraped off by the blade of a cleaner 9 and is collected into a container, whereafter the photosensitive drum 1 is further rotated and the potential of the electrostatic latent image after the transfer of the toner is eliminated by a pre-exposure lamp 116 as pre-exposure means.
The transfer material P separated from the photosensitive drum 1 is sent to a fixing device 10 by a conveying portion 8 comprising a conveying belt, and passes between a fixing roller 113 and a pressing roller 114, whereby the toner image thereon is fixed.
The rotational speed of the fixing roller 113 feeding the transfer material P at this time and the rotational speed of the photosensitive drum 1 are generally set so as to differ from each other, and usually the rotational speed of the fixing roller 113 is made a little lower than the rotational speed of the photosensitive drum 1.
This is for preventing any deviation from being caused in the transfer of the toner image to the trailing end portion of the transfer material P which is still in the transfer portion by the shock when the leading end of the transfer material P rushes into the nip portion between the fixing roller 113 and the pressing roller 114.
The transfer material P on which the toner image has been fixed is then discharged out of the apparatus.
Also, when particularly under high humidity environment, moisture adheres onto the photosensitive drum 1, the electrostatic latent image thereon may be blurred, and in order to prevent this, an on-surface heat generating member 119 as means for evaporating the moisture on the photosensitive drum 1 is disposed in the photosensitive drum 1.
In such an image forming apparatus, in order to stabilize the potential on the photosensitive drum 1, the potential of dark portion and the potential of light portion have been formed on the photosensitive drum 1 and have been read by a potential sensor 111, and the amount of image exposure of the halogen lamp 103 and the amount of charging of the primary charger 2 have been changed and set so that this potential may become predetermined potential, and ideal potential has been formed on the photosensitive drum 1.
The potential has been set by the use of such control with the result that the potential of dark portion and the potential of light portion can be controlled and stable image formation becomes possible, but when the image forming operation is continuously performed, an image memory (ghost image) by residual potential is created by the potential difference between the potential of dark portion and the potential of light portion at a location whereat the photosensitive drum 1 has made a round and an electrostatic latent image has been previously formed.